1. Technical Field
The invention relates generally to electrodeless discharge lamps.
2. Background Art
FIG. 5 shows a schematic view of a conventional electrodeless discharge lamp device that uses microwave energy as an excitation means. The electrodeless discharge lamp device includes a magnetron 1 for generating microwaves of 2.45 GHz, a cavity member 2a, a waveguide 5 for transmitting the microwaves generated by the magnetron 1 into the cavity member 2a, an electrodeless discharge lamp 4 supported within the cavity member 2a by a supporting rod 4a, a motor 6 connected to the supporting rod 4a for rotating the electrodeless discharge lamp 4, and a cooling fan 7 for cooling the magnetron 1. The electrodeless discharge lamp 4 is created by sealing a buffer gas, which is a noble gas, and a luminescent material in a transparent envelope (or discharge tube) such as a quartz glass tube or the like. The cavity member 2a is formed in a cylindrical shape from a conductive material such as a conductive mesh material that does not substantially transmit a microwave but that transmits light. The cavity member 2a is created, for example, by welding a metal mesh plate formed by etching. The cavity member 2a is also provided with a strong electrical connection to the waveguide 5. The space defined by the cavity member 2a and a part of the wall face of the waveguide 5 is called a microwave cavity 2. The microwave cavity 2 communicates with a transmission space inside the waveguide 5 via a power-supply port 3 provided in a wall of the waveguide 5.
The magnetron 1 is positioned with its antenna inserted into the waveguide 5. Microwaves generated by the magnetron 1 are transmitted inside the waveguide 5 from the antenna and are supplied to the microwave cavity 2 through the power-supply port 3. The microwave energy excites the luminescent material within the electrodeless discharge lamp 4, thus allowing the luminescent material to emit light. During the light emission process, the noble gas initially starts to discharge, which causes high temperatures within the electrodeless discharge lamp 4 and a rise in the vapor pressure of the noble gas. As a result, the luminescent material is evaporated and starts to discharge. Subsequently, the vapor pressure of the luminescent material rises and its molecules are excited by the microwave energy to emit light. Consequently, white light with a wide continuous spectrum over the entire visible range is emitted. The light emitted from the electrodeless discharge lamp 4 passes through the cavity member 2a to the outside of the microwave cavity 2.
During operation of the electrodeless discharge lamp 4, the discharge tube wall of the electrodeless discharge lamp 4 tends to have a very high temperature. This is because plasma generated by the microwaves inside the electrodeless discharge lamp 4 spreads inside the tube and, therefore, is present in the vicinity of the inner wall of the electrodeless discharge lamp 4. In this way, the tube wall is exposed to a high temperature. Furthermore, the tube wall of the electrodeless discharge lamp 4 tends to have an uneven temperature distribution because the distribution of the microwave electromagnetic-field strength which determines the plasma density is not three-dimensionally symmetric with respect to the center of the electrodeless discharge lamp 4. Heat transfer due to a convection current inside the tube also contributes to the uneven temperature distribution at the tube wall of the electrodeless discharge lamp 4. The high temperature and uneven temperature distribution in the tube wall of the electrodeless discharge lamp 4 may result in localized high-temperature regions in the material forming the discharge tube wall. Unless the temperature of the discharge tube wall is controlled, these localized high-temperature regions may melt and, thus, result in damage of the discharge tube. In the electrodeless discharge lamp 4 shown in FIG. 5, damage to the discharge tube is prevented by rotating the discharge tube at a moderate speed to obtain a cooling effect which maintains the temperature of the discharge tube substantially uniform.
Various types of luminescent materials are known in the art. However, the selection of luminescent material can affect the temperature of the discharge tube of the electrodeless discharge lamp. For example, the temperature rise in the discharge tube of electrodeless discharge lamps which use sulfur as the luminescent material, e.g., the discharge lamp disclosed in JP 6-132018 A, is considerable. In particular, the microwave energy required to obtain a suitable lamp output results in a temperature which causes the discharge tube to melt easily unless the temperature is controlled. One possible reason for the high temperature is that sulfur has a relatively light atomic weight so that the heat transfer tends to occur inside the electrodeless discharge lamp. Consequently, in the electrodeless discharge lamps which use sulfur as the luminescent material, the discharge tube is initially air-cooled by forcibly blowing cooling air to the discharge tube, and then rotating the discharge tube.
On the other hand, when electrodeless discharge lamps which use indium halide as the luminescent material, e.g., the discharge lamp disclosed in JP 9-120800, are operated under conditions which enable suitable lamp output to be obtained, they do not produce as high a temperature as the electrodeless discharge lamps which use sulfur. Indium-halide electrodeless discharge lamps, however, have a slightly lower luminous efficacy than sulfur electrodeless discharge lamps but are excellent in color rendering. One possible reason for the differences in the temperatures generated by the indium-halide and sulfur electrodeless discharge lamps is that indium halide and sulfur have different gas pressures and molecular weights in operation and, thus, different heat transfer coefficients from the plasma to the tube wall. Therefore, in the electrodeless indium lamp, there is a high possibility that the lamp may be operated without causing a damage to the discharge tube and without employing both the forced-air cooling and the rotating operation of the discharge tube. Actually, when the indium-halide electrodeless lamp is operated without being rotated, the highest temperature in the tube wall is typically not sufficient to damage the discharge tube, even though the temperature in the discharge tube is uneven.
FIG. 3 compares lamp outputs under rotating and non-rotating conditions. In FIG. 3, the horizontal axis indicates supplied microwave power, the vertical axis on the left indicates a luminous flux of a lamp, and the vertical axis on the right indicates the highest temperature of the tube wall. The data a, indicated with X and a broken line, shows the highest temperatures of the tube wall when the lamp is operated without rotating the discharge tube, and the data b, indicated with X and a solid line, shows luminous flux values when the lamp is operated without rotating the discharge tube. The data c, indicated with ∘ and a solid line, shows luminous flux values when the lamp is operated while rotating the discharge tube, and the data d, indicated with ∘ and a broken line, shows the highest temperatures of the tube wall when the lamp is operated while rotating the discharge tube. When the lamp is operated without rotating the discharge tube, the highest temperatures of the tube wall are very high, but do not reach a melting temperature (at least 1100xc2x0 C.) of the discharge tube. The luminous flux values when the discharge tube is rotated does not vary greatly from when the discharge tube is not rotated.
In conventional electrodeless discharge lamp devices, there has been a fear that the mechanism for rotating the discharge tube, for example, the motor 6 shown in FIG. 5, may limit the life span of the lamp device when the lamp device is installed in a severe working environment, e.g., in the open air or the like. Thus, the option of operating an electrodeless discharge lamp device without rotating the discharge tube is quite attractive. However, when the indium-halide electrodeless discharge lamp is operated without being rotated, and the microwave power is increased, discharges tend to be unstable, thus causing flickering. The instability in the discharges occur because the halogen liberated from the indium halide as the vapor pressure inside the tube increases traps electrons in the plasma. To achieve stable lighting, the upper limit of the microwave power has typically been limited, thus limiting the lamp output.
In one aspect, the invention relates to an electrodeless discharge lamp which comprises an envelope filled with a luminescent material to be excited to emit light and a filling material that substantially does not discharge to emit light. The filling material stabilizes a discharge of the luminescent material.
In another aspect, the invention relates to an electrodeless discharge lamp device, which comprises an electrodeless discharge lamp having an envelope filled with a luminescent material to be excited to emit light and a filling material that substantially does not emit light, wherein the filling material stabilizes the discharge of the luminescent material and means for exciting the luminescent material.
In another aspect, the invention relates to an electrodeless discharge lamp device, which comprises an electrodeless discharge lamp having an envelope filled with a luminescent material to be excited to emit light and means for stabilizing discharge of the luminescent material.
In another aspect, the invention relates to a method for producing a stabilized discharge in an electrodeless discharge lamp, which comprises filling an envelope of the electrodeless discharge lamp with a luminescent material and a filling material and exciting the luminescent material to emit light; wherein the filling material stabilizes the emitted light.